LTC3225EDDB#PBF Linear Technology, LTC3225EDDB#PBF Datasheet - Page 7

• Manufacturer Part Number: LTC3225EDDB#PBF
• Manufacturer: Linear Technology

Specifications of LTC3225EDDB#PBF

• Lead Free Status / Rohs Status: Compliant

OPERATION
Once the output voltage is charged to the preset volt-
age threshold, the part shuts down the internal charge
pump and enters into a low current state. In this state,
the LTC3225/LTC3225-1 consume only about 20μA
from the input supply. The current drawn from C
approximately 2μA.
Automatic Cell Balancing
Due to manufacturing tolerances, capacitance and leakage
current can vary from supercapacitor to supercapacitor.
Without the automatic cell balancing scheme used in the
LTC3225/LTC3225-1, the voltages across the supercapaci-
tors could differ from each other and potentially overvoltage
a cell. This can affect the performance and lifetime of a
supercapacitor.
The LTC3225/LTC3225-1 constantly monitor the volt-
age across both supercapacitors while charging. When
the voltage across the supercapacitors is equal, both
capacitors are charged with equal currents. If the voltage
across one supercapacitor is lower than the other, the
lower supercapacitor’s charge current is increased and
the higher supercapacitor’s charge current is decreased.
The greater the difference between the supercapacitor
voltages, the greater the difference in charge current per
capacitor. The charge currents can increase or decrease
as much as 50% to balance the voltage across the su-
percapacitors. When the cell voltages are balanced, the
supercapacitors are charged at a rate of approximately:
If the leakage currents or capacitances of the two superca-
pacitors are mismatched enough that varying the charge
current is not suffi cient to balance their voltages, the
LTC3225/LTC3225-1 stop charging the capacitor with the
higher voltage until they are again balanced. This feature
protects either capacitor from experiencing an overvoltage
condition. Attempting to equalize the voltages using parallel
resistors wastes power, discharges the supercapacitors,
and takes time to equalize the voltages. A 30% capacitance
mismatch leads to a 30% initial voltage difference after
charging. It takes hours to equalize the voltages across
1F supercapacitors using 10k resistors.
I
COUT
=
2
1
•I
VIN
OUT
is
Shutdown Mode
Asserting SHDN low causes the LTC3225/LTC3225-1 to
enter shutdown mode. With the SHDN pin connected to
V
1μA is consumed from the output, allowing the superca-
pacitors to remain charged.
If the input supply is present at V
grounded, the LTC3225/LTC3225-1 draw approximately
1μA of supply current. With the voltage at the C
discharged to 0V, this current drops to less than 1μA. Since
the SHDN pin is a high impedance CMOS input, it should
never be allowed to fl oat.
Output Voltage Programming
The LTC3225/LTC3225-1 have a V
the user to set the output threshold voltage to either 4.8V
or 5.3V for the LTC3225 and 4V or 4.5V for the LTC3225-1
by forcing a low or high at the V
Output Status Indicator (PGOOD)
During shutdown, the PGOOD pin is high impedance. When
the charge cycle starts, an internal N-channel MOSFET
pulls the PGOOD pin to ground. When the output voltage,
V
pin becomes high impedance, but charge current continues
to fl ow until V
When V
age, the PGOOD pin again pulls low.
Current Limit/Thermal Protection
The LTC3225/LTC3225-1 have built-in current limit as well
as overtemperature protection. If the PROG pin is shorted
to ground, a protection circuit automatically shuts off the
internal charge pump. At higher temperatures, or if the
input voltage is high enough to cause excessive self-heat-
ing of the part, the thermal shutdown circuitry shuts down
the charge pump once the junction temperature exceeds
approximately 150°C. It will enable the charge pump once
the junction temperature drops back to approximately
135°C. The LTC3225/LTC3225-1 are able to cycle in and
out of thermal shutdown indefi nitely without latch-up or
damage until the overcurrent condition is removed.
IN
OUT
and the input supply removed or grounded, less than
, is within 6% (typical) of its fi nal value, the PGOOD
OUT
drops 7% below the charge termination volt-
OUT
LTC3225/LTC3225-1
crosses the charge termination voltage.
SEL
SEL
IN
pin respectively.
and the SHDN pin is
input pin that allows
OUT
3225fb
7
pin